1. Field of the Invention
The present invention relates in general to a power-off protection device. In particular, the present invention relates to a power-off protection device to prevent abnormal high-level signal generated during main power shutdown.
2. Description of the Related Art
Recently, most PC products and peripheral devices have adopted switching power, which uses pulse-width modulation (PWM) to control integrated circuits (IC) to act as the power controlling circuit.
FIG. 1 depicts a circuit diagram of a conventional power supply. Pulse-width modulation integrated circuit 10 (PWM IC) outputs a voltage control signal responding to the voltage modulating signal at the input of the PWM IC 10 provided by the CPU 12. The voltage control signal is a high frequency signal. The pulse width of the voltage control signal is adjusted responding to the voltage modulating signal provided by the CPU 12. Then, the voltage control signal is input into a voltage generating circuit 14. The voltage generating circuit 14 generates operating voltage Vop from the power provided by Vcc2 according to the voltage control signal, then the operating voltage Vop is provided to the CPU 12 for operation.
FIG. 2 shows a table of the signals output from CPU 12 (VID0xcx9cVID4) to the PWM IC 10 and the output voltages Vop of the voltage generating circuit 14. The output voltage Vop of the power supply is controlled by the signals input to the PWM IC 10. In FIG. 1, the inputs of the PWM IC 10 (VID0xcx9cVID4) are connected to Vcc1 (3.3V) through pull-up resistor R1. Therefore, when the system is not active, the operating voltage Vop is off while the inputs of the PWM IC 10 (VID0xcx9cVID4) all receive high level signals. In addition, when CPU 10 operates, the PWM IC 10 controls the voltage generating circuit 14 to generate the desired voltage responding to the signals input to the inputs of the PWM IC 10 (VID0xcx9cVID4) from CPU 12.
Generally, when the main power is turned off, Vcc2 drops to low level faster than Vcc1, so the system is turned off successfully. However, when an abrupt Vcc1 drop to low level is faster than Vcc2, at this time, Vcc2 is still at high level, the operating voltage Vop reaches 1.85V (refer to the table in FIG. 2), because the inputs of the PWM IC 10 (VID0xcx9cVID4) all receive low level signals when Vcc2 is still at high level, the voltage generating circuit 14 generates the signal at 1.85V from Vcc2, which damages the CPU.
The present invention provides a power-off protection device, which adds a reference-voltage maintenance device at the input of the PWM IC to delay the PWM IC power dropping to low level, ensuring that the power of the voltage generating circuit drops to low level first. Thus, the situation in the prior art wherein the inputs of the PWM IC receive low level signals and generate excessive high voltage is prevented.
To achieve the above-mentioned object, the present invention provides a power-off protection device for outputting operating power according to a first level signal and a second level signal, which comprises a processor, a modulating circuit, a power generating circuit, and a reference-voltage maintenance device. The processor outputs a voltage modulating signal. The modulating circuit is coupled to the processor and the first level signal to output a voltage control signal responding to the voltage modulating signal. The power generating circuit is coupled to the second level signal and the modulating circuit to generate the operating power from the second level signal according to the voltage control signal. The reference -voltage maintenance device is coupled to the first level signal and the modulating circuit to delay the drop time of the first level signal to confirm that the first level signal is higher than the second level signal when the first level signal and the second level signal are turned off.